1. Field of the Invention
The invention relates to methods and apparatus for producing conductive patterns on a substrate. More particularly, the invention provides a mold on which can be created a thin metallic mirror-image of a conductive pattern which is then printed onto a substrate to produce the desired conductive pattern.
2. Description of the Related Art
Conductive patterns are commonly used in the manufacture of circuit boards that are used in a variety of applications, including providing the circuitry for computers. The known methods of producing printed circuit boards includes several techniques, the most common involving etching of a laminate substrate that is clad with a thin film of copper. The non-etched portion then forms the conductive pattern. In order to produce this pattern, a photoresist layer is first applied to the copper-clad laminate and the photoresist layer is then patterned by exposure to light through an artwork mask. In the case of a negative working photoresist, the areas of photoresist exposed to light are polymerized. The unpolymerized areas are then removed, typically by a chemical solution, exposing copper cladding but leaving the desired conductive pattern protected underneath the polymerized photoresist. The exposed copper is then etched away. After the etching is completed, the remaining polymerized photoresist is chemically removed to expose the copper conductive pattern.
This simple method of producing printed circuit suffers from the disadvantage that etching is not a precisely controllable process. Therefore, when fine lineworks and spacings are required, the lack of etching control becomes increasingly troublesome and may result in a high rejection rate for the completed circuit boards. Moreover, the deposition and etching of large areas of a continuous film produces stresses within the film and coated dielectric material. These stresses promote cracking, spalling of the foil from the dielectric, and dimensional instability. These undesirable effects are intensified when multiple layers of circuit board are used.
In order to produce a laminate with a thin coating of copper metal that can subsequently be etched as described above to provide a conductive pattern on the laminate surface, U.S. Pat. Nos. 4,715,116 and 4,781,991 disclose a process using a polished press plate coated with a layer of electrolytic copper. The coated press plate is laid on top of a suitable base material, such as epoxy resin impregnated glass cloth, and heat and pressure are applied to force the electrolytic layer of copper into the resin of the base material. After the resin is cooled, the press plate is separated from the substrate leaving the copper layer firmly adherent to the base material's surface. The press plate may then be recoated with copper for reuse in applying a copper surface to a suitable polymeric base material. Significantly, these patents do not teach or suggest the application of a conductive pattern to the polymeric base material. Rather, the teachings are restricted to the application of a continuous copper film over the entire surface of a base laminate substrate which might later be etched by conventional techniques to produce a conductive pattern.
U.S. Pat. No. 3,230,163 provides a reusable transfer plate for producing printed circuits on dielectric materials, the transfer plate having on its face a printed circuit flush with an insulating backing layer. In order to produce such a transfer plate, a laminate consisting of a thin foil upper metal layer and a dielectric backing layer of uncured thermosetting resin, is first etched according to any conventional process to form a positive conductive pattern on the metal foil. The conductive pattern is then plated with a hard metal film, such as a chromium film. The resultant product is then pressed in a laminating press which depresses the composite metal pattern into the dielectric while simultaneously curing the resin thereby ensuring intimate bondage of the embedded circuit pattern within the cured dielectric. The exposed surface of the laminated product transfer plate has a hard metal film flush with the surface of the hardened dielectric backing plate. This transfer plate may be used in the production of printed circuits. For instance, the metallic areas of the surface may be replated with electrolytic copper, which does not strongly adhere to the exposed chrome surface. This surface is then pressed onto a layer of resin under laminating conditions to transfer the copper pattern onto the resin layer and produce a conductive pattern thereon. There are several drawbacks to this method, however. For instance, it may be difficult or impossible to plate chromium onto isolated features of the circuitry since there is no plating interconnect for these features. The same problem arises when the electrolytic copper is replated. While an interconnecting bridge is a proposed solution, cutting the bridge mechanically with fine precision would probably require a laser whereas cutting the bridge chemically would require a masking step. In either case the process becomes significantly more complex.
U.S. Pat. No. 4,053,370 discloses a continuous process for the fabrication of printed circuits. In this continuous process, a metal endless strip is first selectively masked with a plating resist to produce a desired circuit pattern. The circuit pattern is then coated with copper by electroplating. The electroplated surface is pressed against an adhesive-coated strip to separate the copper pattern from the stainless steel strip and adhere it to the adhesive coating. The adhesive coated strip may then be bonded to a base strip to produce a circuit board.
U.S. Pat. No. 4,606,787 also uses a type of printing technique for producing a conductive pattern on an insulating material. The method involves first applying a thin flash layer of metal, such as copper, onto a rigid metal or metallized substrate. A thin layer of photoresist material is then applied to the copper surface. A mask is placed over the photoresist layer and the masked surface is exposed to light. Thereafter, the mask is removed and the photoresist is developed thereby producing cavities in the areas where the photoresist dissolves and exposes copper flash. The walls of these cavities are parallel to each other and perpendicular to the metallic substrate. The substrate is then placed in a high impingement speed plating apparatus which electrodeposits copper onto the exposed copper flash to a thickness not exceeding the depth of the cavities. The remaining photoresist layer is then chemically removed to expose a pattern of raised electroplated conductive circuit lines. An insulating layer is then laminated over the conductive circuit pattern under conditions of heat and pressure. When the insulating layer is stripped away from the metallic base, the conductive circuit pattern parts from the substrate and is molded and embedded in the insulating layer. The copper flash layer also adheres to the insulating material and may be removed by conventional etching techniques thereby exposing the conductive circuit pattern to produce the printed circuit board.
U.S. Pat. No. 3,324,014 is also directed to a method for making flush metallic patterns, like those of the 4,606,787 patent. The method includes the steps of coating a temporary support or transfer plate of highly polished relatively hard material with a parting layer that is not strongly adherent to the transfer plate. Thus, the parting layer may consist of dispersions of graphite, finely divided metallic conductor, organic polymers, and the like. A negative resist pattern is then applied over the parting layer and the resist is exposed to light through a mask work to produce a pattern in the resist. The negative resist is then removed and conductive metal is deposited in the spaces left by the removed resist. This forms a conductive pattern on the surface of the transfer plate. A suitable heat-moldable backing material is then molded against the surface of the transfer plate. This backing material is cured in situ by the application of heat and pressure so that it becomes securely bonded to the metallic pattern and completely conforms to the contours of the transfer plate surface. This backing layer or permanent support is then removed from the surface of the assembly taking with it an embedded conductive metallic pattern and the bulk of the parting layer. The parting layer may be removed from the surface of the permanent support by appropriate solvents or physical manipulation leaving behind a permanent support embedded with a printed circuit of conductive metal. In order to reuse the transfer plate for printing, the image of the conductive circuit must again be reproduced on its surface.
U.S. Pat. No. 4,159,222 discloses a method of printing circuitry that also requires the continual renewal or reproduction of a pattern on a polished substrate. The method entails placing a dry film of photoresist on a smooth polished substrate and optionally applying a thin lubricating layer to the exposed photoresist. A mask defining a desired conductive pattern is then brought into contact with the photoresist. This layered assembly is then exposed to light and the photoresist is developed to remove resist and form voids in those regions where the conductive pattern is to be formed. The developed assembly is then subjected to electroplating, to fill the voids formed by the removal of resist. After electroplating, all the remaining resist is removed and a flowable dielectric material is laminated to the electroplated surface of the substrate. The electroplated pattern becomes embedded in the dielectric material so that when the hardened dielectric material is removed, the pattern forms a conductive circuit in its smooth surface. In order to reuse the polished substrate as a "printer," it must again be treated by applying photoresist, exposing the photoresist through a mask, developing the photoresist, and electroplating the voids formed in the resist with a conductor.
U.S. Pat. Nos. 4,889,584 and 4,790,902 also relate to the production of conductor circuit boards by transferring a circuit pattern from an electrically conductive substrate to an insulating substrate by the process of lamination. However, neither of these patents disclose the use of a conductive master mold that may be used for printing conductive patterns onto insulating or dielectric substrates repeatedly, without having to reproduce the conductive pattern on the surface of the master mold by means of photolithography, or otherwise.
What is yet needed in the art is a transfer or printing method and apparatus for producing conductive patterns on dielectric or insulative materials without the need for utilizing photolithography or other techniques to repeatedly reproduce the conductive pattern on the transfer or printing mold. Further, the method and apparatus should desirably be inexpensive while producing fine line resolution so that it may be useful in high density applications.